As techniques for electronic devices, such as a mobile phone, are making progress, demand for flexible printed circuit (hereinafter, frequently abbreviated to “FPC”) boards is rapidly increasing. In recent years, the Product Liability Act has become effective and thus, for securing the safety measures for preventing a fire, it is desired to impart flame retardancy to a resin used in a FPC.
Halogen flame retardants conventionally used, such as an organohalogen compound and a halogen-containing organophosphorus compound, have high flame retardancy effect, but a possibility is pointed out that the halogen flame retardant liberates a halogen during molding or processing to generate corrosive hydrogen halide gas, causing a molding or processing apparatus to suffer corrosion or causing the working atmosphere to be poor. Further, a possibility that the halogen flame retardant generates a gas of hydrogen halide or the like when suffering burning, such as a fire, is pointed out. For this reason, recently, it is desired to use a flame retardant containing no halogen as a substitute for the halogen flame retardant.
A polyester film, particularly a biaxially stretched film using polyethylene terephthalate or polyethylene naphthalate has excellent mechanical properties and excellent heat resistance as well as excellent chemical resistance, and therefore has been widely used as a material for a magnetic tape, a photographic film, a film for packaging, a film for an electronic part, an electrical insulating film, a film for a metal laminate, a protective film, and the like.
As a method for imparting flame retardancy to a polyester resin, a method of copolymerizing a phosphorus compound with a polyester has been studied. For example, JP-A-2007-9111 (PTL 1) discloses that, by using a small amount of a specific carboxyphosphinic acid component among carboxyphosphinic acid components, high flame retardancy can be imparted to polyethylene-2,6-naphthalenedicarboxylate without using other phosphorus compounds in combination. The polyethylene-2,6-naphthalenedicarboxylate film containing the carboxyphosphinic acid compound disclosed in PTL 1 exhibits high flame retardancy in the form of a film. However, with respect to the flame retardancy after the film is processed for an application of a flexible printed circuit or the like, the flame retardancy of the film was unlikely to be reproduced.
As another phosphorus flame retardant, for example, JP-A-2009-179037 (PTL 2) and JP-A-2010-89334 (PTL 3) have proposed a laminate film having laminated on a polyester film a flame retardant layer containing an inorganic metal phosphoric acid derivative, such as a metal phosphinate. However, the proposed laminate film was a technique in which, in view of high burning properties of a porous substrate film due to the voids structure inside of the film, a flame retardant layer using a flame retardant and a curing agent is formed on a substrate film. Further, JP-A-2010-229390 (PTL 4) has a description about a composition having incorporated thereinto an inorganic metal phosphoric acid derivative and a flame-retardant covered electric wire, and these are a technique for imparting flame retardancy to a thermoplastic elastomer resin. The thermoplastic elastomer resin is a resin having a high elongation, and hence encounters almost no problem of a lowering of the physical properties due to the addition of a flame retardant. International Publication No. WO2012/090732 pamphlet (PTL 5) has proposed aflame-retardant polyester film containing a phosphinate or a diphosphinate, and has proposed a flame-retardant biaxially oriented polyester film which is obtained by imparting flame retardancy to a polyester film itself, such as polyalkylene terephthalate or polyalkylene naphthalate, using a phosphorus flame retardant, and which is suppressed in the lowering of a hydrolytic resistance due to the flame retardant.    PTL 1: JP-A-2007-9111    PTL 2: JP-A-2009-179037    PTL 3: JP-A-2010-89334    PTL 4: JP-A-2010-229390    PTL 5: WO2012/090732 pamphlet